Antenna system

ABSTRACT

Embodiments of the invention relate to wireless communications networks, and more specifically to an antenna apparatus for cellular wireless systems. Increasing data capacity of cellular wireless systems places increasing demands on the capacity of the two way connection, known as backhaul, between a cellular base station and a telecommunications network such as the PSTN backhaul, since this is the connection that has to convey the wireless-originating traffic to its destination, often in an entirely different network. Known backhaul links include leased lines, microwave links, optical fiber links or radio resources for relaying backhaul traffic between base stations. The fixed line solutions are expensive to implement and maintain, while the radio solutions antenna configurations that are not ideal for relaying data between base stations. In embodiments of the invention, communication between base stations occurs in a first timeslot by use of a first antenna system and communication between a given base station and a user equipment occurs in a second timeslot using a second antenna system. The benefit of this method is that the first antenna system can be optimized for use in communication between base stations, whereas the second antenna system can be optimized for communication with user equipment which preferably occurs within the area of cellular wireless coverage of the sector served by the second antenna system.

FIELD OF THE INVENTION

The present invention relates generally to antenna systems for wirelesscommunications networks, and more specifically to a method and antennaapparatus relating to wireless backhaul for cellular wireless systems.

BACKGROUND OF THE INVENTION

Mobile telephony systems, in which user equipment such as mobilehandsets communicate via wireless links to a network of base stationsconnected to a telecommunications network, have undergone rapiddevelopment through a number of generations. The initial deployment ofsystems using analogue modulation has been be superseded by secondgeneration digital systems, which are themselves currently beingsuperseded by third generation digital systems such as UMTS and CDMA.Third generation standards provide for a greater throughput of data thanis provided by second generation systems; this trend is continued withthe proposal by the Third Generation Partnership Project of theso-called Long Term Evolution system, often simply called LTE, whichoffers potentially greater capacity still, by the use of wider frequencybands, spectrally efficient modulation techniques and potentially alsothe exploitation of spatially diverse propagation paths to increasecapacity (Multiple In Multiple Out).

Distinct from mobile telephony systems, wireless access systems havealso undergone development, initially aimed at providing the “last mile”(or thereabouts) connection between user equipment at a subscriber'spremises and the public switched telephone network (PSTN). Such userequipment is typically a terminal to which a telephone or computer isconnected, and with early systems there was no provision for mobility orroaming of the user equipment between base stations. However, the WiMaxstandard (IEEE 802.16) has provided a means for such terminals toconnect to the PSTN via high data rate wireless access systems.

Whilst WiMax and LTE have evolved via different routes, both can becharacterised as high capacity wireless data systems that serve asimilar purpose, typically using similar technology, and in additionboth are deployed in a cellular layout as cellular wireless systems.Typically such cellular wireless systems comprise user equipment such asmobile telephony handsets or wireless terminals, a number of basestations, each potentially communicating over what are termed accesslinks with many user equipments located in a coverage area known as acell, and a two way connection, known as backhaul, between each basestation and a telecommunications network such as the PSTN.

As the data capacity of cellular wireless systems increases, this inturn places increasing demands on the capacity of the backhaul, sincethis is the connection that has to convey the wireless-originatingtraffic to its destination, often in an entirely different network. Forearlier generations of cellular wireless systems, the backhaul has beenprovided by one or more connections leased from anothertelecommunications operator (where such a connection exists near to thebase station); however, in view of the increasing data rates, the numberof leased lines that is required is also increasing. Consequently, theoperational expense associated with adopting multiple leased lines hasalso increased, making this a potentially expensive option for highcapacity systems.

As an alternative to leased lines, dedicated backhaul links can beprovided by a variety of methods including microwave links or opticalfibre links. However each of these methods of backhaul has associatedcosts. Dedicated fibre links can be expensive in terms of capitalexpense due mainly to the cost of the civil works in installation, andthis problem is especially acute in urban areas. Microwave links alsoinvolve the capital expense of equipment and require expert installationdue to narrow beam widths leading to the requirement for precisealignment of antennas.

As an alternative to the provision of a dedicated backhaul link for eachindividual base station, it is possible to use the radio resource of thecellular wireless system to relay backhaul traffic from one base stationto another. Typically, the base station using the cellular radioresource for backhaul is a small low power base station with anomnidirectional antenna known as a relay node. Such a system can be usedto extend the area of cellular wireless coverage beyond the area ofcoverage of conventional base stations that are already equipped with adedicated backhaul.

FIG. 1 shows a conventional relay node operating within a cellularwireless network; the operation may for example be in accordance withIEEE 802.16j. A user equipment 5 b is in communication with a relay nodebase station 3. As already mentioned, the relay node typically employsan omnidirectional antenna giving a uniform radiation pattern 15 inazimuth. As the relay node 3 is not provided with a backhaul linkseparate from the cellular wireless resource, the relay node isallocated radio resource timeslots for use relaying backhaul data to andfrom the adjacent base station 1 which is itself connected by microwavelink to a microwave station 7 and thence to a telecommunications networksuch as the public switched telephone network The base station 1 in thisexample employs a conventional tri-cellular coverage scheme; threeantennas are each connected to a radio transceiver at the base stationand the radiation pattern consists of three lobes 11 a, 11 b and 11 c. Auser equipment 5 a is shown in communication with the base station 1 viaantenna pattern lobe 11 c. It should be noted that the antennas employedby the base station 1 to give the tri-sectored coverage scheme areoptimized to give coverage within this particular cellular scheme; as aresult there are regions between the antenna lobes 11 a, 11 b, 11 c withlittle coverage from the base station, as these areas are designed to becovered by a neighbouring base station. So it can be seen that the basestation antennas are not designed to give even coverage over 360 degreesfrom an individual base station. Also, the antennas on the tri-cellularbase station 1 are given a deliberate down-tilt of several degrees togive good coverage within the sector in question while minimizinginterference with other base stations. As a result, this antennaarrangement may not be ideal for communication with a relay node 3outside the normal area of coverage of the base station 1, as, dependingon its location, the relay node may fall in a null between the lobes inthe tri-cellular radiation pattern and in addition the down-tilt of theantennas may reduce gain beyond a certain distance from the basestation.

FIG. 2 shows a time frame structure allocating timeslots alternately toaccess 17 a . . . 17 d and backhaul 19 a . . . 19 c. Typically, all ofthe access payload data will be relayed by the backhaul link; if thespectral efficiency of the backhaul and access links is the same, thenthe access and backhaul timeslots will occupy approximately equalamounts of time. There may be a significant reduction in capacityavailable in the access links to the user equipment due to the need toreserve timeslots for backhaul. This problem is exacerbated in the IEEE802.16j scheme, where the allocation of timeslots to backhaul islocalized around the area of the relay node.

Hence it can be seen that backhaul links for high capacity cellularwireless systems can present a significant expense; to mitigate this,the cellular wireless resource can be used to relay backhaul links fromone base station to another, but when employed in conventionalarrangements, this incurs significant limitations to data capacity andrestrictions on the positioning of base stations.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a system for transceiving signals in a cellular wirelesscommunications network, the cellular wireless communications networkcomprising a first base station, a second base station and a userequipment terminal, the system being arranged to receive a messageapportioning timeslots for particular types of transmission, saidmessage being received at the first base station, the second basestation and said user equipment terminal, wherein the system is arrangedto transceive first signals between said first base station and saidsecond base station in a first timeslot on the basis of timeslotsapportioned in said message, and to transceive second signals betweensaid first base station and said user equipment terminal in a secondtimeslot on the basis of timeslots apportioned in said message, thesecond signals occupying at least part of the frequency band occupied bythe first signals,

wherein the system comprises:

a first antenna system of a first type arranged to transceive said firstsignals at the first base station; and

a second antenna system of a second type, different from the first typeof antenna system, arranged to transceive said second signals at saidthe base station.

Thus in embodiments of the invention, communication between the firstbase station and the second base station occurs in a first timeslot byuse of a first antenna system and communication between the first basestation and a user equipment occurs in a second timeslot using a secondantenna system. The benefit of this method is that the first antennasystem can be optimised for use in communication with the second basestation, whereas the second antenna system can be optimised forcommunication with user equipment which preferably occurs within thearea of cellular wireless coverage of the sector served by the secondantenna.

Preferably, the radiation pattern of the first antenna is narrower inazimuth than that of the second antenna. A narrower beam is beneficialsince this increases the gain of the beam and reduces the transmittedand received interference signal power. A stronger ratio of signal tonoise and interference received in the link between base stationsenables a higher data rate so that a smaller share of the availabletimeslots is required for the link between the first and second basestations. It is feasible that the first antenna can have a narrower beamin azimuth than the second antenna, since the first antenna receivessignals only from the second base station—which is fixed inlocation—whereas the second antenna has to receive signals from userequipments, which are mobile and can potentially be present in any partof the sector of the second antenna.

Conveniently, the first or second antenna system is selected forconnection to a transceiver by a radio frequency switch; this has thebenefit that a single radio transceiver and feeder cable can be sharedbetween transceiving signals in the first and second timeslots.

Preferably, the control of the radio frequency switch is by means of thedetection of a power characteristic at the output of the transceiver andby means of decoding of a message representing the switching point withrespect to the power characteristic; this advantageously removes theneed for a dedicated control cable between the transceiver and theswitch and thus reduces the costs of the transceiving system.

In one arrangement, the first antenna system comprises an antenna arrayand the second antenna system comprises a subset of the antenna array;this has the advantage of minimising the size of the combination of thefirst and the second antenna systems.

Conveniently, the first and second antenna systems are implemented byapplying alternative amplitude and phase weighting values to amulti-element beamformer. On reception, a multi-element beamformerreceives signals from an array of antenna elements and modifies theamplitude and phase characteristic of the signal from each antennaelement and combines the signals to form a single output. Ontransmission, a multi-element beamformer receives a single input signal,splits the signal into multiple components, and modifies the amplitudeand phase characteristics of each signal transmitted to each antennaelement. This has the benefit that the antenna pattern of the array ofantenna elements can be controlled by the application of weightingvalues in a programmable manner without physical modification to theantenna array.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional relay node incommunication with a tri-cellular base station;

FIG. 2 is a schematic diagram showing a conventional frame structureenabling timesharing between backhaul and access components within alocal area;

FIG. 3 is a schematic diagram showing a transceiving system according toan embodiment of the invention;

FIG. 4 is a schematic diagram showing access and backhaul beam patternsgenerated by the transceiving system of FIG. 3 in elevation;

FIG. 5 is a schematic diagram showing an implementation of an antennaselection system forming part of the transceiving system of FIG. 3;

FIG. 6 is a schematic diagram showing an alternative implementation ofan antenna selection system forming part of the transceiving system ofFIG. 3;

FIG. 7 is a schematic diagram showing a yet further implementation of anantenna system forming part of the transceiving system of FIG. 3;

FIG. 8 is a schematic diagram showing a yet further implementation of anantenna system forming part of the transceiving system of FIG. 3;

FIG. 9 is a diagram showing a network of transceiving systems of FIG. 3,each implementing switched antennas according to an embodiment of theinvention between access and backhaul modes in a single frequencynetwork;

FIG. 10 is a diagram showing a network of transceiving systems of FIG.3, each implementing switched antennas according to an embodiment of theinvention between access and backhaul modes for a network employingthree frequency bands; and

FIG. 11 is a diagram showing allocation of frequencies to backhaul linksin a network of transceiving systems of FIG. 3, configured to employthree frequency bands.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention is directed to methods and apparatusthat provide backhaul by using the cellular wireless resource within acellular wireless system. For clarity, the methods and apparatus aredescribed in the context of a high speed packet data system such asIEEE802.16 (WiMax) or LTE, but it will be appreciated that this is byway of example and that the methods and apparatus described are notlimited to this example.

FIG. 3 shows a first embodiment of the invention. As in the case ofconventional arrangements, a user equipment 5 b is in communication witha relay node base station 3, the relay node producing an omnidirectionalradiation pattern 15. Backhaul from the relay node 3 is provided by alink 18 to a base station 1 which itself has a microwave link to amicrowave station 7 and thence to a telecommunications network 9. In thesystem illustrated in FIG. 1, the backhaul link 18 from the relay node 3to the base station 1 is via lobe 11 a of the radiation pattern producedby the base station 1, which is optimized for access connections, suchas that between the base station 1 and the user equipment indicated byreference numeral 5 a. In the embodiment illustrated in FIG. 3, however,the backhaul link between the relay node base station 3 and the basestation 1 is carried by a different lobe 17 of the radiation pattern ofthe base station 1; this is because the base station 1 uses a differentantenna system when backhaul messages are transmitted and received thanit does when access messages are transmitted and received.

As a result, the link 18 in the direction of the relay node can beoptimized, by using an antenna beam pointed directly at the relay node.The radiation pattern of the beam 17 can be narrower in azimuth thanthat of the beam 11 a used for access, since it is not necessary to givecoverage over the breadth of a given sector; this allows the gain of thebeam to be increased, potentially improving the signal to noise andinterference ratio of the link to the relay node by increasing thereceived signal strength and reducing the probability of interferencefalling within the beam. An improved signal to noise plus interferenceratio enables the data rate of the backhaul link to be increased bymeans of adaptive modulation and coding; as a result, the proportion oftime allocated to the backhaul link can be reduced, thereby increasingthe potential capacity of the access links and providing more time foruse in access than is available in conventional systems. The signal tonoise plus interference ratio may be further improved by the use of anadditional radiation pattern lobe at the relay node.

FIG. 4 shows the backhaul beam 17 and access beam 11 a at base station 1in elevation. It can be seen that the access beam 11 a has considerabledowntilt relative to the backhaul beam 17; this is undesirable for usein backhaul if the base station with which communication is desired isat the extremes of the coverage area or mounted on a tower. Accordingly,it is preferable not to apply downtilt to the backhaul beam 17. Also,the backhaul beam 17 is somewhat broader in elevation that is the accessbeam 11 a; unlike the downtilt this can be tolerated because thereduction in azimuth beamwidth of the backhaul beam 17 relative to theaccess beam 11 a gives an improvement in gain that more than compensatesfor a broadening of the beam in elevation. An advantage to be had from abroadening of the backhaul beam in elevation is that the antenna sizecan be reduced, with consequent reduction in wind loading and towerrental fees that may be charged per square foot of antenna area.

FIG. 5 shows a block diagram of a switched antenna system according toan embodiment of the invention. The antenna system comprises a backhaulantenna and an access antenna 19, 21, together with associated controlcomponents, as will now be described in more detail. The access antenna19 is shown as a vertical array of antenna elements 20 a; this is atypical structure that gives a broad beam in azimuth and a narrow beamin elevation. On reception, the signals received from the antennaelements 20 a are combined together, and on transmission the transmittedsignals are split between the antenna elements. The backhaul antenna 21shown is also constructed from an array of antenna elements 20 b; in apreferred arrangement the backhaul antenna 21 is four elements in widthrather than a single element wide, as was the case for the accessantenna, giving the access antenna a relatively narrower beam inazimuth. However, the skilled person will appreciate that the backhaulantenna 21 could alternatively have a width equivalent to two, three, ormore elements and still provide a relatively narrower beam in azimuth.The increased gain associated with the narrower beamwidth in azimuthallows a reduction in the height of the antenna, increasing thebeamwidth in elevation, as discussed.

A single pole double throw (SPDT) switch 23 is used to select thebackhaul antenna 21 within a backhaul timeslot and the access antenna 19within an access timeslot. Typically, the switch 23 would be a PIN(P-type Intrinsic N-type) diode switch designed to carry the high powerof the transmitted signals.

In one arrangement the antennas 19, 21, switch 23 and associated switchcontrol components 35, 37, 39, 41 are mounted at the top of an antennatower while a radio transceiver 27 is mounted at the bottom of thetower, for ease of maintenance. The transceiver 27 is connected to thetower top components by a feed cable 25. It is generally costly toinstall additional cables between the bottom and the top of a tower;hence it is preferable to position the antenna switch 23 at the top ofthe tower, to remove the need for a second feed cable that would berequired if the switch were positioned at the bottom of the tower. It issimilarly undesirable to install a control cable between the transceiver27 and the switch 23. A consequence of the avoidance of the installationof additional cables is that the control of the switch 23 is preferablyarranged to be derived from signals present on the feed cable 25.Typically, no existing interface to the feed cables 25 is available thatis sufficiently fast to operate at the speed of the backhaul/accessswitching; accordingly, a method is used whereby a message on anexisting antenna control interface, such as the industry standard AISGinterface, is used to define switching points with respect to a counter.A counter is then synchronized to the detected power envelope of thetransmit/receive waveform.

The operation of the switch control is thus as follows. Signals arecoupled from the feeder cable 25 using a coupler 35 and AISG messagesare decoded in an AISG decoder 39. These messages represent the requiredantenna switching points in terms of the count on a counter. An envelopedetector 37 detects the transmit/receive power envelope and passes thedetected waveform to the switch control 41. The switch control 41synchronises a conventional flywheel counter with the power envelopesignal, such that a given count on the counter consistently represents agiven phase of the transmit/receive cycle. A comparator switches thestate of the switch at the count values indicated by the AISG message.

FIG. 5 represents the transmission and reception of signals usingantennas with a single state of polarization; transmission and receptionof signals on orthogonal polarisations can be carried out by employingantennas with dual polarization outputs and duplicating the switch 23,feed cable 25 and transceiver 27; one set of switch control circuitry35, 37, 39, 41 could be used to control the switches 23 on bothpolarizations. Similarly, any of the embodiments can be implemented indual polar form by the suitable duplication of signal paths.

FIG. 6 shows an alternative embodiment of a switched antenna system. Theaccess antenna 19 and backhaul antenna 21 are similar to those in thesystem of FIG. 5, but in this embodiment a radio frequency switch is notrequired; instead two duplicate transceivers 27 a, 27 b are utilised.Typically the transceivers 27 a, 27 b would be sited at the top of theantenna tower close to the antennas 19, 21, and the switching betweenaccess and backhaul mode is then carried out by a digital multiplexer43. The need for a feeder cable that is low loss and typically heavy andexpensive is thus removed, and a lightweight and cheap optical fibrecable 45 could instead be used as a backhaul connection. For thisembodiment to be economically viable, the requirement is that it shouldbe cheaper to use two transceivers that share the same RF channel thanone specially modified transceiver intended to operate in a different RFfrequency band (as in conventional microwave backhaul systems), and inaddition, the transceivers should be reliable enough to be placed at thetop of the antenna tower, given the cost associated with maintenance atthat location.

FIG. 7 shows a further alternative embodiment of a switched antennasystem. In this system, an antenna array 21 is used with some elementsswitched in or out of use according to whether a backhaul antenna 21 oraccess antenna 19 is required. As shown, the array has two columns often elements 20 a. For access mode, a single column 19 is connectedthrough a combiner/splitter 33 to a transceiver 27. For backhaul mode,the second column is switched in, thereby narrowing the beam. A phaseshifter 29 determines the relative phase between the signals on thefirst and second antenna array columns referred to the transceiver 27.Adjustment of the phase shifter 29 will steer the backhaul beam inazimuth; the adjustment could be an electronic or a purely mechanicalpath length adjustment. The single pole single throw (On/Off) switch 31could be implemented using PIN diodes, and the control technique couldbe similar to that used in the embodiment of FIG. 5.

FIG. 8 shows another alternative embodiment of the switched antennasystem. In this arrangement, the antenna system is embodied by amulti-element beamformer, comprising an array of weight values 47 a . .. 47 n that are used to control the amplitude and phase of signalstransmitted and received by an array of antenna elements. A beamformercontroller 51 controls the application of weights to be suitable forproducing the antenna patterns appropriate for backhaul or access mode.The application of weights to antenna elements using a beamformer iswell known in the field of phased array radar and electronic beamsteering generally. The benefit of the electronic beam steeringtechnique is the degree of control it gives over the precise shape ofthe antenna beam, thereby allowing the optimization of the link gain orother system parameters. In addition, nulls may be steered in thedirection of interference sources to maximize the signal to noise plusinterference ratio.

FIG. 9 shows an application of an embodiment of the invention, in whicha single frequency band is used (a so-called N=1 frequency re-usescheme), and in which backhaul is provided from a second base station 3is a conventional tri-cellular base station of similar characteristicsto those of the first base station 1 and is similarly typically mountedon a tower. As a result, antennas with different radiation patterns forbackhaul and access modes are used on both of the base stations. Theconventional tri-cellular arrangement is shown by hexagonal cells 13 a .. . . 13 f, each of which receives wireless coverage from antennapattern 11 a . . . 11 f. A feature of a tri-cellular arrangement is thata neighbouring base station will tend to fall into a null in the antennapatterns of a given base station; this is illustrated by examination ofthe beam from the second base station 3 indicated by reference numeral11 e. The radiation patterns 11 a, 11 c of the antennas on the side ofthe first base station 1 facing the second base station 3 have nullstowards the second base station 3. It is likely, therefore, that anylink between the first and second base stations 1, 3 using accessantennas would produce a link with a poor signal quality due to poorantenna gain. This example illustrates the benefit of using alternativeantennas for access: it can be seen that antenna patterns with referencenumerals 17 a and 17 b can be made to align and therefore produce a linkthat would be expected to exhibit a high signal to noise ratio. Such alink could exploit adaptive modulation and coding techniques to give ahigh data rate, thereby requiring a smaller share of transmission timethan would be the case if the data rate on the link were lower.

FIG. 10 illustrates a variation of the scheme illustrated in FIG. 9, inwhich three frequency bands are used: a so-called “N=3 frequency re-use”scheme. In this configuration, each base station will provide coverageto three cells, each in a different frequency band: the access beam 11 eemanating from the second base station 3 pointing towards the first basestation 1 operates at frequency band f2, whereas the access beamoperating at f2 from the first base station 1 is indicated by referencenumerals 11 b and faces directly away from the second base station 3.This is an example of a case which particularly benefits from theapplication of switched antennas between access and backhaul modes; thebeams indicated by reference numerals 17 a and 17 b can be set up tooperate at the same frequency so they can communicate with each other,in this case at frequency f2. Note that this arrangement may requireswitching between an access antenna on one side of a given antenna towerand a backhaul antenna on the other side of the tower: as between beamsindicated by reference numerals 17 a and 11 b.

FIG. 11 illustrates a frequency re-use pattern that could be used formultiple backhaul links in an N=3 frequency reuse scheme according tofigure 10. The Figure shows theoretical signal outputs of a particularconfiguration of first base stations (including those referenced byparts 1 a and 1 b) and second base stations (including those referencedby parts 3 a and 3 d): due to the tri-cellular layout illustrated, thebackhaul links would appear to line up, thereby introducing thepotential for interference from a distant base station. In practice thedeployment of antennas is unlikely to match that of a theoretical grid,but it is nevertheless possible for a backhaul beam to experienceinterference in this way. More specifically, from the Figure it can beseen that if base stations at positions indicated by 1 b and 3 d were touse frequency f2 for the backhaul link 18 b the transmissions from thesecond base station labeled 3 d would be directly in line with thebackhaul link 18 a of the first base station labeled 1 a and vice versa.Advantageously, however, the configuration of FIG. 11 provides amechanism for selecting the frequency of operation of respectivebackhaul links, thereby effectively adjusting the distance between linksthat are in line with one another and operating at the same frequency.Indeed, in the arrangement shown, the base stations have been configuredso as to ensure that these backhaul links 18 a, 18 b, 18 c do notoperate at the same frequency.

In the description above relating to various configurations for theantenna selection system, the backhaul and access signals are describedas being transceived within the same frequency band. It will beappreciated that this covers at least two different arrangements: afirst in which the respective signals use the same channel (implyingtime division only), and a second in which the respective signals usedifferent, e.g. adjacent, channels (implying frequency division as wellas time division). Whilst the above embodiments relate to the former,time-division only, arrangement, the scope of the invention covers botharrangements. Indeed, in order to accommodate the latter arrangement,the antenna selection system would additionally include a frequencyswitching component.

The above embodiments are to be understood as illustrative examples ofthe invention, and other embodiments are envisaged. It is to beunderstood that any feature described in relation to any one embodimentmay be used alone, or in combination with other features described, andmay also be used in combination with one or more features of any otherof the embodiments, or any combination of any other of the embodiments.Furthermore, equivalents and modifications not described above may alsobe employed without departing from the scope of the invention, which isdefined in the accompanying claims.

1. A system for transceiving signals in a cellular wirelesscommunications network, the cellular wireless communications networkcomprising a first base station, a second base station and a userequipment terminal, the system being arranged to receive a messageapportioning timeslots for particular types of transmission, saidmessage being received at the first base station, the second basestation and said user equipment terminal, wherein the system is arrangedto transceive first signals between the first base station and thesecond base station in a first timeslot on the basis of timeslotsapportioned in said message, and to transceive second signals betweenthe first base station and said user equipment terminal in a secondtimeslot on the basis of timeslots apportioned in said message, thesecond signals occupying at least part of the frequency band occupied bythe first signals, wherein the system comprises: a first antenna systemof a first type arranged to transceive said first signals at the firstbase station; and a second antenna system of a second type, differentfrom the first type of antenna system, arranged to transceive saidsecond signals at the first base station.
 2. A system according to claim1, comprising a radio frequency switch for connecting the first antennasystem to a transceiver for the duration of said first timeslot and forconnecting the second antenna system to the transceiver for the durationof said second timeslot.
 3. A system according to claim 1, wherein thefirst antenna system comprises an antenna array having a plurality ofantenna elements, and the second antenna system comprises a sub-set ofsaid antenna elements.
 4. A system according to claim 3, including afurther radio frequency switch for use in selecting said sub-set ofantenna elements.
 5. A system according to claim 1, each said first andsecond antenna systems having a beamwidth associated therewith, whereinthe beamwidth in azimuth of said first antenna system is narrower thanthat of the second antenna system.
 6. A system according to claim 5,wherein the beamwidth in elevation of the first antenna system isbroader than that of the second antenna system.
 7. A system according toclaim 1, each said first and second antenna systems having a radiationpattern associated therewith, wherein the peak of the radiation patternof the first antenna system is at a substantially different azimuthbearing than that of the second antenna system.
 8. A system according toclaim 1, wherein the first antenna system and the second antenna systemeach comprises a multi-element beamformer adapted to apply a respectiveset of amplitude and phase weighting values in respective timeslots. 9.A method of configuring a cellular wireless communications networkcomprising a first base station, a second base station and a userequipment terminal, the method comprising: receiving a messageapportioning timeslots for particular types of transmission, the messagebeing received at the first base station, the second base station andsaid user equipment terminal; transceiving first signals between thefirst base station and the second base station in a first timeslotapportioned in said message; and transceiving second signals between thefirst base station and said user equipment terminal in a second timeslotapportioned in said message, the second signals occupying at least partof the frequency band occupied by the first signals, wherein the methodfurther comprises: transceiving said first signals by means of a firstantenna system of a first type at the first base station; andtransceiving said second signals by means of a second antenna system ofa second type at the first base station, the second type of antennasystem being different from the first type of antenna system.
 10. Amethod according to claim 9, comprising connecting the first antennasystem to a transceiver for the duration of said first timeslot andconnecting the second antenna system to the transceiver for the durationof said second timeslot so as to selectively transceive said firstsignals within said first timeslot and to transceive said second signalswithin said second timeslot.
 11. A method according to claim 10,comprising: decoding a message indicating a switching point with respectto a count value on a counter; detecting a characteristic representing apower output of the transceiver as a function of time; synchronising thecounter with the characteristic; and switching between the first antennasystem and the second antenna system in dependence on the count valueand the switching point.
 12. A method according to claim 11, in whichthe first antenna system comprises an antenna array having a pluralityof antenna elements and the second antenna system comprises a sub-set ofsaid antenna elements, the method further comprising switching betweensaid sub-set of antenna elements in dependence on the count value andthe switching point.
 13. A method according to claim 12, includingapplying a first set of amplitude and phase weighting values during saidfirst time slot and applying a second, different, set of amplitude andphase weighting values during a said second timeslot.